The chore of the genealogist/Family Historian is to find a document, read the document, evaluate the document, and pull out information that seems to agree with other information gathered previously. Or, perhaps this is the first historical document on which you will see your family. In that case, you will want to investigate other sources such as other census info, death and marriage information, land records, etc. You won't really know the value of the first piece of evidence until you have compared it with other pieces.

The reason that a census page cannot be primary source in and of itself:

We don't know who was home the day that the enumerator came round. It may have been an older child or an old aunt who was visiting. Even if mom or dad was there, they may have been illiterate--as many people were back in the day--literacy took a back seat to survival! I know that "illiteracy" doesn't equal stupidity, but if you are illiterate you may not have paid quite as much attention to exact dates, exact spellings, etc. Comments, anyone? Agree? Disagree?

The enumerator was a human being and made very human errors. He may have had terrible penmanship or the best possible handwriting. He may have been hot, tired, and hungry and in a hurry. He may have been hard of hearing? The list of maybe's is a long one.

Family members may have become separated, married off, or deceased, so don't expect it to show the perfect record on the census. Families were very much on the move in rural American and their lives weren't static models.